1. The Field of the Invention
The present invention relates to compositions and methods for manufacturing thermoplastic starch compositions and articles made therefrom. More particularly, the present invention relates to thermoplastic starch compositions that include a particulate filler component. The thermoplastic starch compositions may optionally include one or more additional thermoplastic polymers blended therewith and fibers for reinforcement.
2. The Relevant Technology
A. Sheets, Containers, and Other Articles Made From Paper, Plastic, Glass and Metal.
Materials such as paper, paperboard, plastic, polystyrene, and metals are presently used in enormous quantity as printed materials, labels, mats, and in the manufacture of other articles such as containers, separators, dividers, envelopes, lids, tops, cans, and other packaging materials. Advanced processing and packaging techniques presently allow an enormous variety of liquid and solid goods to be stored, packaged, or shipped while being protected from harmful elements.
Containers and other packaging materials protect goods from environmental influences and distribution damage, particularly from chemical and physical influences. Packaging helps protect an enormous variety of goods from gases, moisture, light, microorganisms, vermin, physical shock, crushing forces, vibration, leaking, or spilling. Some packaging materials also provide a medium for the dissemination of information to the consumer, such as the origin of manufacture, contents, advertising, instructions, brand identification, and pricing.
Typically, most containers and cups (including disposable containers) are made from paper, paperboard, plastic, polystyrene, glass and metal materials. Each year over 100 billion aluminum cans, billions of glass bottles and thousands of tons of paper and plastic are used in storing and dispensing soft drinks, juices, processed foods, grains, beer, etc. Outside of the food and beverage industry, packaging containers (and especially disposable containers made from such materials are ubiquitous. Paper for printing, writing, and photocopying, as well as magazines, newspapers, books, wrappers, and other flat items made primarily from tree derived paper sheets are also manufactured each year in enormous quantities. In the United States alone, approximately 51/2 million tons of paper are consumed each year for packaging purposes, which represents only about 15% of the total annual domestic paper production.
Recently there has been a debate as to which of these materials (e.g., paper, paperboard, plastic, polystyrene, or metal) is most damaging to the environment. Consciousness-raising organizations have convinced many people to substitute one material for another in order to be more environmentally "correct." The debate often misses the point that each of these materials has its own unique environmental weaknesses. One material may appear superior to another when viewed in light of a particular environmental problem, while ignoring different, often larger, problems associated with the supposedly preferred material (e.g., whereas paper is more biodegradable than plastics and polystyrene, paper is far more polluting to the environment to manufacture).
The debate should not be directed to which of these materials is more or less harmful to the environment, but rather toward asking whether an alternative material can be developed which will solve most, if not all, of the various environmental problems associated with each of these presently used materials.
B. Starch.
Starch is a plentiful, inexpensive and renewable material that is found in a large variety of plant sources, such as grains, tubers, fruits, and the like. In many cases, starch is discarded as an unwanted byproduct of food processing. However, because starch is readily biodegradable it does not persist in the environment as a harmful material when disposed of. Perhaps the only harm that starch might cause is that it can put unwanted nutrients into the water or soil into which it is discarded, which could attract and facilitate the proliferation of certain unwanted organisms. It is this quality as a nutrient, though, that greatly facilitates the breakdown and elimination of starch from the environment.
Because of the biodegradable nature of starch many have attempted to incorporate starch into a variety of materials in order to improve the environmental desirability of such materials. Starch has been incorporated into multi-component compositions in various forms, including as a filler, binder, or as a constituent within thermoplastic polymer blends. In addition, some have attempted to utilize starch alone as a thermoplastic material, although with limited success due to the tendency of starch to form retrograde crystallization products upon resolidifying, which crystallization products often lack appropriate mechanical properties.
Starch may be added as an inert filler, typically in its native, unmodified state, which is a generally water-insoluble, granular material. In such cases, the starch granules will normally behave as any other solid particulate filler and will contribute little, if any, in terms of improving the mechanical properties of the resulting material. Alternatively, starch that has been gelatinized, dried, and then ground into a powder may also be added as a particulate filler. Although starch may be added as a filler, its more interesting and technologically challenging uses have been in the area of using starch as a binder, as a thermoplastically processible constituent within thermoplastic polymer blends, and as a thermoplastic material by itself.
Although the alternative uses of starch as a water-soluble binder or as a thermoplastic material generally require significantly different compositional formulations and process conditions in order to successfully process them as intended, they have the common requirement that the native starch granules must in some way be transformed or altered from being in a granular or particulate state to being in a molten or plastic state, such as be dissolution or gelation within a solvent or by being heated to form a starch melt. Because native starch has a melting point that approaches the decomposition temperature, it is virtually impossible to form a starch melt without the addition of plasticizers, solvents or other components that allow the starch to become molten, solvated or otherwise liquified into a plastic state at a temperature that is safely below the decomposition temperature.
Starch can be used as a "binder" in order to glue or otherwise adhere other solid constituents together to form a heterogenous mixture of different components. At some point before or during the molding phase, the starch is typically dissolved or gelatinized in an appropriate solvent, such as water, in order for it to become a liquid or gel. This allows the initially granular starch to become a flowable or plastic material into which the other components can be dispersed. Upon resolidification of the gelatinized starch, typically by removing enough of the water by evaporation so that the starch recrystallizes or otherwise dries out, the starch forms a solid or semi-solid binding matrix that can bind the remaining components together. Examples of patents that teach the use of starch as a binder and, in particular, processes for molding articles from aqueous starch mixtures include U.S. Pat. No. 5,660,900 to Andersen et al.; U.S. Pat. No. 5,683,772 to Andersen et al.; U.S. Pat. No. 5,709,827 to Andersen et al.; U.S. Pat. No. 5,868,824; and U.S. Pat. No. 5,376,320 to Tiefenbacher et al. For purposes of disclosing compositions, methods, and systems for molding aqueous starch mixtures that are subsequently dried so as to form a binding matrix of dried starch which binds together discrete solid materials such as fibers and/or particulate fillers, the foregoing patents are incorporated herein by specific reference.
Related to the process of molding aqueous starch mixtures is the formation of sheets having properties similar to conventional paper and paperboard by methods that do not require the use and subsequent removal of the huge quantities of water required in conventional paper-making processes. Examples of compositions, processes, and systems; for continuously manufacturing sheets from aqueous starch-based mixtures in a manner that does not utilize conventional drainage or dewatering are set forth in U.S. Pat. No. 5,736,209 to Andersen et al. and U.S. Pat. No. 5,810,961. For purposes of disclosing composition, methods and systems for the formation of sheets from aqueous starch-based mixtures, the foregoing patents are incorporated herein by specific reference.
Many have also attempted to use starch as a thermoplastic material, either alone or as a component within thermoplastic blends. Native starch does not typically behave as a thermoplastic material by itself but must be heated in the presence of some kind of plasticizer. Typically, the plasticizer must be a liquid (at least when raised to the resulting chemically compatible with starch, which is itself highly polar due to the existence of hydroxyl groups on approximately half of the carbon atoms. Typically, plasticizers used to assist the formation of starch melts have been either highly volatile liquids at the melting point, such as water, or low volatile liquids, such as glycerin.
Starch melts using water as the plasticizing solvent have been referred in the art as "destructurized starch". Starch is said to be "destructurized" because it ceases to be a solid granular particulate as found in its native state. Moreover, it is said to be "destructurized" because the dissolution or melting of starch in the presence of water is an irreversible process. Starch that has been dissolved into or melted in the presence of water can never return to its native, granular state. Upon resolidification of a melt of destructurized starch, typically by cooling below its melting or softening point, it will yield an essentially amorphous or semicrystalline starch material that is self-supporting or "form stable", but only so long as the water content is kept above at least 5% by weight of the starch and water mixture during the entire process including during cooling, preferably above at least 10%. Otherwise, the starch will tend to recrystallize into a brittle material instead of forming a more amorphous and less brittle solid.
The use of "destructurized starch" as a commercial thermoplastic material has been limited for a number of reasons, including difficulty in processing, poor long term mechanical properties, high sensitivity to fluctuations in ambient moisture, including poor dimensional stability, and the difficulty of forming homogeneous blends of destructurized starch with more hydrophobic polymers that are less sensitive to fluctuations in moisture. Examples of patents that disclose the manufacture of "destructurized starch" and blends of destructurized starch and other polymers include U.S. Pat. No. 4,673,438 to Wittwer et al.; U.S. Pat. No. 4,900,361 to Sachetto et al.; U.S. Pat. No. 5,095,054 to Lay et al.; U.S. Pat. No. 5,256,711 to Tokiwa et al.; U.S. Pat. No. 5,275,774 to Bahr et al.; U.S. Pat. No. 5,382,611 to Stepto et al; U.S. Pat. No. 5,405,564 to Stepto et al.; and U.S. Pat. No. 5,427,614 to Wittwer et al. For purposes of disclosing compositions and methods for manufacturing "destructurized starch" compositions, including blends of "destructurized starch" and other polymers, the foregoing patents are incorporated herein by specific reference.
Others have taught that it is preferable to greatly reduce the amount of water in starch melts by replacing the water inherently found in starch with an appropriate low volatile plasticizer capable of causing starch to form a thermoplastic melt below its decomposition temperature, such as glycerin, polyalkylene oxides, mono- and diacetates of glycerin, sorbitol, other sugar alcohols, and citrates. This allows for improved processability, greater mechanical strength, better dimensional stability over time, and greater ease in blending the starch melt with other polymers compared to "destructurized starch". Thermoplastic starch materials in which most or all of the water has been replaced by a low volatile plasticizer, either before or during processing, have been variously referred to as "thermoplastically processible starch" and "thermoplastic starch".
Water can be removed before processing by using starch that has been predried so as to remove at least a portion of the natural water content. Alternatively, water can removed during processing by degassing or venting the molten mixture, such as by means of an extruder equipped with venting or degassing means. Examples of patents that teach the manufacture of thermoplastically processible starch, including blends of thermoplastic starch and other polymers, include U.S. Pat. No. 5,362,777 to Tomka; U.S. Pat. No. 5,314,934 to Tomka; U.S. Pat. No. 5,280,055 to Tomka; U.S. Pat. No. 5,415,827 to Tomka; U.S. Pat. No. 5,525,281 to Lorcks et al.; U.S. Pat. No. 5,663,216 to Tomka; U.S. Pat. No. 5,705,536 to Tomka; U.S. Pat. No. 5,770,137 to Lorcks et al.; and U.S. Pat. No. 5,844,023 to Tomka. For purposes of disclosing compositions and methods for the manufacture of thermoplastic starch compositions, blends thereof, and articles of manufacture therefrom, the foregoing patents are incorporated herein by specific reference.
Still others have manufactured thermoplastic starch blends in which native starch is initially blended with a small quantity of water together and a less volatile plasticizer such as glycerin in order to form starch melts that are subjected to a degassing procedure prior to cooling and solidification in order to remove substantially all of the water therefrom. Examples of such patents include U.S. Pat. No. 5,412,005 to Bastioli et al.; U.S. Pat. No. 5,280,055 to Bastioli et al.; U.S. Pat. No. 5,288,765 to Bastioli et al.; U.S. Pat. No. 5,262,458 to Bastioli et al.; 5,462,980 to Bastioli et al.; and U.S. Pat. No. 5,512,378 to Bastioli et al.
Regardless of whether water or another plasticizer is used to form a starch melt, all destructurized and thermoplastic starch materials have been limited in the market place by the inherent mechanical limitations of starch melts and their relatively high cost. Although many have attempted for years to discover the "perfect" starch/polymer blend that would yield an environmentally sound polymer while, at the same time, fulfilling desired mechanical and cost criteria, such a combination has not yet been achieved. The reason for this is that the emphasis has been on finding the optimal synthetic polymer or mixture of synthetic polymers and other admixtures in order to thereby "optimize" the properties of the starch/polymer blend. One drawback is that most of the synthetic polymers and other admixtures are themselves significantly more expensive than starch, which tends to increase the cost of such polymer blends compared to starch melts. Another drawback is that such additives will only be able to marginally alter the mechanical properties of the starch/polymer blends when viewed from a materials science perspective.
In spite of the inherent economic limitations associated with thermoplastic starch blends, the focus of researchers has remained rigidly fixed on the goal of finding the "perfect" thermoplastic polymer or other admixture that will yield the "perfect" starch-polymer blend. Although extremely inexpensive fillers such as naturally occurring mineral materials have been added to concrete and other building materials, their use as an inexpensive filler within destructurized or thermoplastic starch systems has been largely ignored. Although the aforementioned U.S. Pat. No. 5,362,777 to Tomka discloses the inclusion of an inorganic filler, such filler component is limited to concentrations of 3% or less by weight. Likewise, the aforementioned U.S. Pat. No. 5,427,614 to Wittwer et al. discloses the use of an inorganic "texturizing agent" having a concentration of 1% or less. At such low concentrations, inorganic fillers will only have a marginal impact on the cost and mechanical characteristics of the thermoplastic or destructurized starch materials disclosed therein.
Based on the foregoing, what are needed are improved thermoplastic starch compositions and methods for manufacturing low cost, environmentally friendly sheets, films, and molded articles having appropriate mechanical properties similar to, e.g., paper. paperboard, polystyrene, other plastics, metal sheets, and the like.
It would be a significant improvement in the art if such thermoplastic starch compositions allowed for the formation of a variety of containers or other articles using existing manufacturing equipment and techniques presently used to form articles from paper, polymer films, or moldable plastic materials.
It would yet be an advancement in the art if such environmentally friendly thermoplastic starch compositions could be formed from compositions that only included a fraction of the starch content compared to other starch-based compositions presently being utilized.
It would be a significant improvement in the art if such thermoplastic starch compositions yielded articles that were readily biodegradable and/or degradable into substances commonly found in the earth.
From a practical point of view, it would be a significant improvement to provide thermoplastic starch compositions and methods which allowed for the manufacture of sheets, containers, and other articles at a cost that was comparable to or even lower than the cost of existing methods of manufacturing articles from paper, plastics, or other materials.
It would be a further advancement in the art to provide thermoplastic starch compositions and methods which allowed for the inclusion of less organic polymer materials while overcoming many of the problems associated with compositions based on starch melts.
It would also be a tremendous advancement in the art to provide thermoplastic starch compositions and methods which allowed for the inclusion of significant quantities of an inorganic filler and, optionally fibrous materials, both organic and inorganic, within such starch compositions.
In addition, it would be an advancement in the art to provide thermoplastic starch compositions that had improved physical properties, such as increased thermal stability, increased modulus of elasticity, compressive strength, and toughness compared to conventional thermoplastic starch compositions.
Such thermoplastic starch compositions and methods for manufacturing starch-based sheets, films articles therefrom, and molded articles are disclosed and claimed herein.